The present invention relates generally to elements conveying optical radiation, and specifically to semiconductor devices conveying optical radiation.
Semiconductor optical amplifiers (SOAs) are well known in the art as relatively inexpensive gain elements, and are used, inter alia, either singly or in cascade to compensate for reductions in power in optical networks. The reductions in power may be caused, for example, by losses generated by relatively long lengths of fibers, or by using elements such as one-to-many splitters within the network.
Unfortunately, SOAs suffer from a number of problems caused by inherent gain non-linearities and relatively long gain recovery times. For example, transmitting a long series of ones via an SOA can cause gain saturation in the SOA, so that a following zero bit is subject to gain compression. Similarly, a long series of zero bits causes a following one bit to be subject to an undesired high gain. Effects such as these lead to problems such as inter-symbol interference between bits of a single data stream, and also to cross-talk between bits in pluralities of data streams such as when wavelength division multiplexing is used.
One solution to the problems is to operate the SOA at a substantially average constant output power level, which is preferably set below, a 3 dB saturation level of the SOA. An example of this approach is described in an article titled xe2x80x9c160 Tb/sec DWDM transmission over 160 km of standard fiber using a cascade of SOA""s,xe2x80x9d by L. H. Spiekman et al., in the post deadline paper PD2-7 of the 25th European Conference of Optical Communications (1999), which is incorporated herein by reference. The article describes monitoring the output of an SOA.
In implementing waveguides in semiconductor devices such as SOAs, parasitic reflections at input and output facets of the device can degrade the performance of the device. The parasitic reflections can be substantially reduced by anti-reflection coating of the facets, and also by implementing the waveguides so they are tilted, i.e., non-orthogonal, to the facets. An example of this approach is described in an article titled xe2x80x9cFabrication and Performance of 1.5 xcexcm Traveling Wave Amplifiers with Angled Facets,xe2x80x9d by C. E. Zah et al., in the Vol. 21 (1987) issue of Electronics Letters, which is incorporated herein by reference.
A further problem in implementing waveguides in semiconductors is that curvature of the waveguide causes radiation leakage from the waveguide. The leaked radiation is a function of the radius of curvature; it is also a function of the refractive index ratio of the semiconducting material implementing the waveguide and the surrounding substrate within which the waveguide is implemented. As is shown in an article titled xe2x80x9cLow loss III-V semiconductor optical waveguidesxe2x80x9d by R. J. Deri et al., in the Vol QE-27 (1991) issue of IEEE Journal of Quantum Electronics, which is incorporated herein by reference, the leakage can be controlled by careful choice of radius of curvature and the refractive indices of the media involved.
It is an object of some aspects of the present invention to provide a method and apparatus for monitoring a power output of an optical amplifier.
It is a further object of some aspects of the present invention to provide a method and apparatus for improving gain characteristics of the optical amplifier.
It is another object of some aspects of the present invention to provide a method and apparatus for monitoring optical radiation traversing a waveguide.
In some preferred embodiments of the present invention, an optical amplifier is implemented in a semiconductor device. The device comprises a waveguide, through which light is output from the amplifier. The waveguide comprises a curved section. By setting values for its radius of curvature, length, and refractive index step, the curved section is formed so that a known, substantially fixed, fraction of the amplifier output leaks from the section. A photodetector is implemented in the semiconductor device in a position where it captures the leaked light. The photodetector generates an output directly related to a power of the leaked light and thus to the amplifier output. Thus, unlike other amplifiers and output monitors known in the art, the light amplifier and output monitor are implemented together in one monolithic semiconductor device.
In some preferred embodiments of the present invention, the output from the photodetector is used in a negative feedback loop for controlling a bias current to the optical amplifier. By controlling the bias current responsive to the output light power, effects due to gain saturation and long gain recovery time of the amplifier are substantially reduced.
In other preferred embodiments of the present invention, an optical radiation monitor is implemented as a monolithic device comprising a curved waveguide coupled, as described above, to a photodetector, absent an optical amplifier. Output from the photodetector provides a measure of the optical radiation travelling through the waveguide.
There is therefore provided, according to a preferred embodiment of the present invention, an optical radiation monitor, including:
an input port which receives the optical radiation;
a waveguide, coupled to the input port so as to receive the optical radiation Therefrom and adapted to leak a predetermined fraction of the optical radiation; and
a photodetector which receives at least some of the leaked optical radiation and which generates a monitoring signal responsive thereto.
Preferably, the waveguide and photodetector are integrally formed on a single, common substrate of semiconductor material.
Preferably, the waveguide includes a curved waveguide having a predetermined radius of curvature and a refractive index different from a refractive index of the substrate.
Preferably, the input port is coupled to an input waveguide and the waveguide is coupled to an output waveguide, wherein the input and output waveguides are integrally formed on the single, common substrate.
There is further provided, according to a preferred embodiment of the present invention, an optical amplifier, including:
an optical gain region which is adapted to output amplified optical radiation responsive to a current injected into the section;
a waveguide, coupled to receive the amplified optical radiation, and adapted to leak a predetermined fraction of the amplified optical radiation; and
a photodetector which receives at least some of the leaked optical radiation and which generates a monitoring signal responsive thereto, indicative of a performance characteristic of the optical gain region.
Preferably, the gain region, the waveguide and photodetector are integrally formed on a single, common substrate of semiconductor material.
Further preferably, the apparatus includes a feedback control, which is coupled to receive the monitoring signal and to alter the injected current responsive to the monitoring signal.
Preferably, the waveguide includes a curved waveguide having a predetermined radius of curvature and a refractive index different from a refractive index of the substrate.
Preferably, the performance characteristic includes at least one of a group of parameters comprising an output power level of the optical gain region, and a pulse length, an extinction ratio, and a spontaneous emission level of optical radiation therein.
There is further provided, according to a preferred embodiment of the present invention, a method for monitoring optical radiation, including:
inputting the optical radiation into a waveguide;
arranging the waveguide so that a predetermined fraction of the optical radiation leaks from the waveguide; and
measuring the leaked optical radiation so as to monitor a characteristic of the radiation in the waveguide.
Preferably, measuring the leaked optical radiation includes providing a photodetector to perform the measurement, and arranging the waveguide includes integrally forming the waveguide and photodetector on a single, common substrate of semiconductor material.
Preferably, arranging the waveguide includes forming a curved waveguide in a substrate, the curved waveguide having a predetermined radius of curvature and a refractive index different from a refractive index of the substrate.
There is further provided, according to a preferred embodiment of the present invention, a method for amplifying optical radiation, including:
injecting current into an optical gain region so as to engender amplification of the optical radiation in the region;
coupling the amplified optical radiation into a waveguide;
arranging the waveguide so that a predetermined fraction of the amplified optical radiation leaks from the waveguide; and
generating a monitoring signal, indicative of a performance characteristic of the optical gain region, responsive to the leaked optical radiation.
Preferably, the method includes varying the injected current responsive to the monitoring signal.
Preferably, the method includes providing a photodetector to measure the monitoring signal, and integrally forming the optical gain region, the waveguide, and the photodetector on a single, common substrate of semiconductor material.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawing, in which: